1. J. H. Lacy, J. M. Achtermann, and E. Serabyn, Astrophys. J. 380, 71 (1991).
2. D. A. Roberts and W. M. Goss, Astrophys. J. Suppl. Ser. 86, 133 (1993).
3. B. Vollmer and W. J. Duschl, arXiv:astro-ph/9904096 (1999).
4. V. V. Lyakhovich, A.M. Fridman, and O. V. Khoruzhii, in Unstable Processes in the Universe, Collected vol. (Kosmosinform, Moscow, 1994), p. 194 [in Russian].
5. A. M. Fridman, O. V. Khoruzhii, V. V. Lyakhovich, and L. Ozernoy, in Galactic and Solar System Optic Astrometry: Observation and Application, Ed. by L. V. Morisson and G. F. Gilmore (Cambridge Univ., 1994), p. 239.